The present invention relates to a process for starting up a reaction for the polymerization of an olefin in the gas phase carried out in a fluidized bed reactor using a catalyst based on chromium oxide.
When starting up a reaction for the polymerization of an olefin in the gas phase employing a catalyst based on chromium oxide, formation of agglomerates and/or of fine particles of polymer is regularly observed. Furthermore, it may also happen that polymer having an undesirable quality is produced, for example by formation of a polymer having an undesired melt flow index. These phenomena appear in particular after having begun to introduce the catalyst into the fluidized bed and up to the time when a certain amount of polymer is produced, in particular up to the time when a stable level of production of polymer is reached, that is to say the desired maximum level of production.
It has also been observed that, during the start-up period, the instability of the polymerization conditions often results in uncontrolled reactions and the formation of agglomerates and/or of fine particles of polymer.
A novel process for starting up a reaction for the polymerization of an olefin in the gas phase carried out in a fluidized bed reactor has now been found which makes it possible to solve the abovementioned problems. More particularly, the process of the invention makes it possible to start up a polymerization reaction with very limited, indeed zero, formation of agglomerates. Furthermore, it is also the case that no formation of fine particles of polymer is observed.
According to the present invention there is provided a process for starting up a reaction for the polymerization of an olefin in the gas phase carried out using a catalyst based on chromium oxide in a fluidized bed reactor through which moves a reaction gas mixture containing the olefin, characterized in that in the start-up of the reaction the olefin is introduced into the reactor until the desired production of polymer is reached by means of a two-stage process, during the first stage of which the introduction of the olefin is carried out so as to maintain a constant partial pressure of olefin in the reactor and during the second stage, the introduction of olefin is regulated at a constant flow rate.